The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 100 63 014.6 filed on Dec. 16, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a process for the breaking and cleaning of contaminant-containing raw paper stocks, e.g., recovered paper, in which a disintegration apparatus is used to produce a contaminant-containing suspension. The contaminant-containing suspension is fed to a closed coarse screen, from which a cleaned flow fraction is drawn off as well as an overflow fraction that contains an increased amount of contaminants.
2. Discussion of Background Information
As is known, raw paper stocks that contain a smaller or larger percentage of contaminants are used to a great extent for paper manufacture. Such contaminants are predominantly pieces of plastic and plastic foils, wires, pieces of metal, broken glass, sand, etc. The contaminants should, of course, be removed as completely as possible, i.e., to protect the machines used for the processing from wear and because of the quality demands on the paper produced later.
Processes have proved effective in which the raw paper stock is first broken at a solids content of above 8 percent. This means that after mixing with water, the raw paper stock is dispersed by mechanical processing, whereby the raw paper stocks disintegrate as a result of their swelling, while a large part of the contaminants retain their firmness. Thus, e.g., plastic foils remain in relatively large pieces, and the paper webs can be broken down into individual fibers or lumps. When this state is reached, further water is added so that a suspension is formed that is, as a rule, capable of being pumped and contains a relatively high percentage of contaminants. As a result, the subsequent screening apparatus receives a strongly contaminated suspension. Fiber constituents that no longer contain any coarse contaminants can be drawn off as accepted stock fractions through the screen contained therein, so that the suspension remaining behind inside the screening apparatus, i.e., that is not carried away as accepted stock, is constantly enriched with contaminants. This can lead to operating trouble and loss of quality due to the reduced screening effect. If such screening apparatuses are rinsed in cycles, the continuity of the manufacturing process suffers. Thus relatively large machines are required.
The present invention provides a process that renders possible a secure trouble-free operation even in the presence of quite large amounts of contaminants.
According to the invention, the overflow fraction is drawn off from the coarse screen during at least about 80% of the operating time. In this manner, a volumetric mass flow of the overflow fraction is adjusted to at least about 15% and maximally about 50% of a volumetric mass flow of the inflow into the coarse screen. Further, the overflow fraction is fed to a further screening device from which the collected contaminants are sluiced out discontinuously as an overflow fraction to form a further cleaned flow fraction.
The operation of the plant implementing the process is substantially improved by the described measures. It is in fact possible to draw off an already well-cleaned accepted stock at the coarse screen, which stock is preferably available continuously, e.g., as a rule uninterruptedly. During the sluicing of the subsequent screening device, the coarse screen takes over the entire production. Although the accepted stock of the screening device used subsequently is not produced continuously, it is quantitatively less. Its screening quality can be set by appropriate parameters so that it can be conducted forwards.
The instant invention is directed to a process for the disintegration and cleaning of contaminant-containing raw paper stocks. The process includes producing a contaminant-containing suspension, feeding the contaminant-containing suspension to a closed coarse screen, drawing of a cleaned flow fraction from the closed coarse screen, and drawing off an overflow fraction containing an increased amount of contaminants from the coarse screen during at least about 80% of an operating time. The process also includes adjusting a volumetric mass flow of the overflow fraction to at least about 15% and maximally about 50% of a volumetric mass flow of an inflow into the coarse screen, and feeding the overflow fraction to a screening device, in which contaminants collected are sluiced out discontinuously as a further overflow fraction, thereby forming a further cleaned flow fraction.
In accordance with a feature of the instant invention, the contaminant-containing suspension can be produced in a disintegration apparatus. Further, the contaminant-containing raw paper stocks may include recovered paper.
According to another feature of the invention, the overflow fraction can be drawn off from the coarse screen at least until no more contaminants are sluiced from the screening device.
According to still another feature of the present invention, the flow fraction may be continuously drawn off from the coarse screen.
Further, the sluicing of contaminants out of the screening device may include halting the feeding of the overflow fraction into the screening device and, while the feeding is halted, pumping a rinsing jet into the screening device. The rinsing jet can cause the contaminants to flow off from the screening device as the further overflow fraction.
In accordance with a further feature of the invention, during the sluicing, the entire residue collected in the screening device can be removed.
A discharge of the further cleaned flow fraction from the screening device may be blocked from the coarse screen while the sluicing is taking place.
The further cleaned flow fraction from the screening device can be combined with the cleaned flow fraction from the coarse screen.
Moreover, the further overflow fraction of the screening device can be conducted into a screening drum in which a reject fraction is formed as an overflow.
According to another feature of the invention, the flow fraction through the screening drum may be returned into the disintegration apparatus.
Further, the further overflow fraction from the screening device can be thickened in a press.
The volumetric mass flow of the overflow fraction can be adjusted at the coarse screen to correspond to between about 20% and about 30% of the volumetric mass flow of the inflow into the coarse screen.
Further, a mass flow of fibrous material in the further overflow fraction may be adjusted at the screening device so that it amounts to about 20% to about 30% of a mass flow of fibrous material in an inflow to the screening device.
In accordance with still another feature of the instant invention, the coarse screen may include a screen with an aperture size of maximally about 4 mm.
According to a further feature of the invention, the coarse screen can include a flat screen kept clear by a moving screen scraper.
Further still, the contaminant-containing suspension may be centrally introduced into the coarse screen.
According to a still further feature of the invention, heavy particles can be separated at the coarse screen via a heavy particle sluice.
Still further, a breaking apparatus may be arranged upstream of the coarse screen to produce the contaminant-containing suspension, and no contaminants are removed between the breaking apparatus and the coarse screen.
The process may also include rejecting coarse contaminants at a screen perforation of maximally about 30 mm diameter between disintegration apparatus producing the contaminant-containing suspension and the coarse screen. The coarse contaminants may be removed with a rotating screening drum having a perforation size of maximally about 30 mm.
According to still another feature of the present invention, a disintegration apparatus produces the contaminant-containing suspension.
According to a further feature of the invention, the disintegration apparatus can include a dissolving tank. An axial continuation of the dissolving tank may be structured and arranged as a screening part. Further, the axial continuation of the dissolving tank can be utilized as a screening drum. Still further, the dissolving tank can be operated such that a continuous flow of disintegrated material accumulates, and the accumulated disintegrated material is diluted with dilution water to produce the contaminant-containing suspension.
In accordance with a still further feature of the instant invention, the disintegration apparatus can include a high-consistency pulper. Further, the high-consistency pulper can be operated discontinuously and that at the end of a breaking cycle a desired consistency for the coarse screen is established by adding dilution water.
According to still another feature of the invention, an inside volume on a feed side of the coarse screen can be at least about 15% of a volume that the volumetric mass flow of the cleaned flow fraction fills in one minute.
In accordance with still yet another feature of the present invention, an inside volume on an feed side of the screening device may be at least about 30% of a volume that a volumetric mass flow of the further cleaned flow fraction fills in one minute.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.